This invention pertains to devices employed for making up threaded joints on tubular goods (tubing and casing) used in oil and gas wells. Joint integrity is of utmost importance since a leak in any one joint of high pressure tubing can necessitate killing the well and removing and replacing the tubing string. Such removal and replacement is expensive. In other situations leakage in tubing joints can cause loss of the entire well.
Power tools have been in general use for a number of years for making up threaded tubular goods. The term bucking unit is generally applied to makeup machines used to install couplings on pipe usually in a pipe mill or threading facility. These units are used to make up and buck up couplings on pipe lying in the horizontal position. The bucking unit assembly includes a drive unit which imparts a rotary torque to a coupling and the backup unit which holds the end of the pipe in position and provides the necessary backup torque. Typical of such units is the Model 811 bucking unit manufactured by Bowen Tools, Inc., Peck-o-matic Division in Houston. Chapter 6, especially pages 6-7 of the 1982 catalog disclose the major components of the bucking unit.
The term "power tongs" is generally applied to makeup machines used to connect joints or lengths of pipe together to form continuous strings as successive joints are added and the assembled pipe is lowered into the well. Rotary power for making up the threaded connection is provided by the power tongs. A backup device is normally mounted below the power tongs to hold the adjacent pipe stationary as the power tongs are actuated.
Both the drive unit and the backup unit employ gripping devices or jaws which move inwardly to contact the relatively smooth surface of the pipe or coupling and impart sufficient forces thereto to transmit torque from the driving means to the members being threadedly joined.
Various mechanisms have been used to actuate the jaws. For example, in U.S. Reissue Pat. No. 31,699, sliding heads 32 and 32a hold jaws 92. The heads 32 are guided by channels 110. The heads are mounted with a roller 114 which interacts with a cam ring 90 such that rotation of the cam ring results in inward displacement of the rollers 114 out of depressions 180 and 180a toward the pipe to be gripped. In U.S. Pat. No. 4,084,453 link members 134 and 136 are pivotally mounted to a pin on a drag plate. The forward dies 144 and 146 are radially displaced when rollers 152 and 154 are displaced due to the rotation of the cam ring.
U.S. Pat. No. 4,089,240 discloses a similar design which uses three dies on each member 134. Still other similar designs are disclosed in U.S. Pat. No. 4,350,062. U.S. Pat. No. 4,334,444 shows actuation of die carriers by displacement of a roller mounted to the die carriers upon rotation of a cam ring. A pair of dies having short circumferential dimensions are used with each jaw block. The design in U.S. Pat. No. 4,215,602 is very similar. Dies having longer arcuate lengths have been used as illustrated in literature published by South Coast Tong and Rental, Inc. of Lafayette, La. and sold under the SUREGRIP trademark. A pair of such dies are claimed to grip around 220.degree. of a pipe. The manufacturer claims that such design reduces the possibility of crushing in high torque situations which are encountered in working over problem wells. This design employs rows of parallel teeth. Claims are further made in sales literature that the jaws of this design effectively distribute the load over a much larger area of the tubing thereby assuring less penetration into the pipe. Experience has shown that such load distribution characteristics in high torque applications have not been realized.
U.S. Pat. No. 4,593,584 also illustrates extended length dies which claim to grip greater than 230.degree. around the pipe. U.S. Pat. No. 4,372,026 discloses a plurality of pinion gears connected to cam elements which move into and out of contact with the pipe upon rotation of a ring gear. The cam surfaces engage the pipe as opposed to the jaw designs of the other patents discussed above.
U.S. Pat. No. 4,334,444 shows the general arrangement of a power tong which employs a rotatable cam ring or base or bowl and cam dogs and dies. As the cam ring is rotated, the dies are urged inward by the cam surface until the dies contact the pipe and bite into the surface to permit the transmission of tangential torsional forces through friction. The contact between the dies and the pipe is essentially a line contact. In essence, each die makes an initial contact along a line parallel to the longitudinal axis of the pipe. The bulk of the initial forces transmitted from the jaw to the pipe occur along this line hence the term line contact. In practice, due to the fact that the pipe deflects under the heavy forces involved and due to the fact that the jaw teeth penetrate into the surface of the pipe, the contact area is spread to a limited extent and forms a narrow band of contact.
In any case, localized gouging of the pipe is undesirable as is permanent deformation of the pipe.
Applying the principles of mechanics and strength of materials, it can be shown that the normal forces applied to the pipe at the points where the die contacts the pipe are quite large with the normal load on the leading die in the direction of rotation, being considerably larger than the normal load on the trailing die. The result is that the pipe is deformed considerably during the process of making up a joint and the jaw teeth can cause severe penetration of the pipe surface. Deformation during makeup and the residual permanent deformation adversely effects the integrity of the joint.
The advantage of the camming design disclosed in U.S. Pat. No. 4,334,444 using a pair of opposed jaw blocks is that one jaw size can accommodate a range of pipe sizes. The range can be further increased by inserting thicker dies in the same jaw blocks.
The wrap around die design illustrated in U.S. Pat. No. 4,593,584 reduces the deflection of the pipe in an apparent attempt to spread the load over a larger area of the pipe circumference. However, there still exists a large variation from the toe or leading edge of the die to the heel or trailing edge of the die. The tooth penetration at the leading edge of the die is somewhat reduced; however, the forces can still be sufficient to cause scoring or the pipe wall and in some instances permanent deformation.
Reissue Pat. No. 31,699 illustrates a guided jaw. Jaws are placed in slots in an effort to equalize the forces over the arc of contact. However, new frictional forces evolve between the jaw block and the slots requiring additional applied forces. Clearance necessary for freedom of movement of the jaw block within the guides permits the jaw block to move sufficiently to cause the dies at the leading edge to bite deeply into the pipe surface. This phenomenon is true whether dies having a small circumferential length as shown in Reissue Pat. No. 31,699 are used or whether the wrap around type dies such as those shown in U.S. Pat. No. 4,593,584 are used.
U.S. Pat. No. 4,350,062 illustrates an offset cam dog design.
U.S. Pat. No. 4,372,026 illustrates a gear actuated design. A ring gear is rotated by means of external gear drive or chain drive. As the internal gear rotates it acts upon the gear segments of the jaw mechanism causing the jaw blocks to rotate from the retracted position to engage the pipe. As soon as the die makes contact with the pipe, the friction forces cause the jaw block to wedge between the pipe and the ring gear. Thereafter the ring gear, the jaws and the pipe rotate together as a unit. Normally five jaws are used for smaller pipe and up to ten for larger diameter pipes. The normal loads on the pipe are all essentially equal, hence there is less distortion of the pipe as compared to the cam dog designs described above and less damage to the pipe surface.
An object of this invention is to provide a jaw block and die combination design to provide uniform distribution of forces along the face of the dies to minimize biting into the pipe by the die and deflection of the pipe under heavy loads.